Re: [Xen-devel] [PATCH] Yield to VCPU hcall, spinlock yielding

[Orran Y Krieger <okrieg@xxxxxxxxxx>]

habanero@xxxxxxxxxxxxxxxxxxxxxxx wrote on 06/09/2005
02:59:56 PM:

> I take it this assumes nothing else is running on that domain?  I
agree 
> in this ideal scenerio, it would work (assumming the app threads waiting
> for the lock are not busy waiting), but what about apps which have
other 
> threads which don't wait on that particular lock, keeping the other
cpus 
> busy, or a domain with more than one app running?  In those cases
the 
> load balance would probably not happen, and I would think we need
more 
> than just the high prio kernel thread to move the desired task to
a 
> active virtual cpu.

The key place where this came up in our internal discussions
was running big HPC apps and big data bases...  In both those cases,
the dominant situation is your ideal scenario.  In a multiprogrammed
environment, you have the problem even without a hypervisor, i.e., a process
holding a lock can be preempted by another process.   Currently, Linux
doesn't do any schedular aware synchronizaiton, so presumably this is not
viewed as a problem.  If some day this is viewed as a problem, then
the preemption upcall could be used by whatever mechanism linux chooses
to implement.  I am not convinced that we need a special purpose solution
for virtualization...
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